There have previously been described polarizers for changing polarization from linear to circular in operation with electromagnetic waves having a frequency within a frequency band and having an angle of incidence within a range of angles. However, the usable incidence angle range of prior polarizers has been limited. For example, a linearly-polarized phased-array antenna may be arranged to electronically scan a radiated beam to any angle from zero to 70 degrees off broadside in any plane. Conversion of such linear polarization to circular polarization may be accomplished by a polarizer placed in front of the phased-array, however the performance of prior polarizers has degraded substantially over such a range of incidence angles.
More specifically, prior designs of circular polarizers may incorporate several spaced arrays of susceptance elements which are oriented at 45 degrees to an incident linear polarization for broadside incidence of an incident wave (i.e., a zero degree angle of incidence). However, at larger angles of incidence the polarizer elements will no longer have an orientation close to 45 degrees relative to the electric field vector of the incident wave. As a result, the polarizer performance degrades as the angle of incidence increases (for example, the axial ratio increases, so that the resulting polarization is no longer circular) and the polarizer becomes unusable beyond a limited range of incidence angles. Thus, performance of a typical prior such polarizer may degrade rapidly beyond a zero to 35 degree angle of incidence range. Also, the susceptance of such polarizer elements changes as the incidence angle is changed. These changes in susceptance, which are likely to be different for E-plane incidence and H-plane incidence, also limit the usable incidence angle range for prior polarizers.
Basic wide-band linear to circular polarizer concepts were described by D. S. Lerner in "A Wave Polarization Converter for Circular Polarization", IEEE Trans. Antennas and Propagation, Vol. AP-13, pp. 3-7, Jan. 1965. Further developments of meander-line elements for use in such polarizers were described by Young, Robinson and Hacking in "Meander-Line Polarizer", IEEE Trans. Antennas and Propagation, Vol. AP-21, pp. 376-378, May 1973 and by Chu and Lee in "Analytical Model of a Multilayered Meander-Line Polarizer Plate with Normal and Oblique Plane-Wave Incidence", IEEE Trans Antennas and Propagation, Vol AP-35, pp. 652-661, June 1987. The latter two articles discuss the theory and design of meander-lines, which are polarization changing elements in the form of continuous zig-zag conductive patterns supported on thin dielectric sheets. As is well known, such polarizer elements appear essentially capacitive for an incident electric field perpendicular to length of such meander-lines and appear essentially inductive for an incident electric field parallel to the length of the meander-lines. The meander-line approach can provide improved axial ratio and improved frequency band performance. However, as described and shown by Chu and Lee, for a polarizer using known design techniques both the transmission coefficient and the input VSWR began to degrade rapidly for scan angles greater than about 30 degrees (see page 658 and FIGS. 6(a) and 6(b) of the referenced Chu and Lee article). In their Conclusion, at page 659 Chu and Lee particularly point out that: "It is shown that because the powers contained in the E-type and H-type modes of the incident wave are not equal for oblique incidence, there will be degradation in axial ratio when the meander-line polarizer is used in the oblique incidence case."
FIG. 1 shows an array of polarizer elements in the form of a parallel array 10 of meander-line elements 14 oriented at 45 degrees from the horizontal and vertical. Polarizer element arrays of this type, formed as a thin metallic pattern, are used in prior polarizers. As described in the references cited above, a basic metallic pattern, such as array 10, mounted on one surface of a thin dielectric: support sheet has typically been used in polarizers incorporating three or more of such array sheets maintained in spaced parallel relation by relatively thick foam intermediate layers positioned between the array sheets. In such configurations, the thin support sheets are specified to provide required structural support of FIG. 1 type arrays, while minimizing the operative effect of the inclusion of the dielectric material necessitated for such support purposes. Similarly, in such prior configurations, the thicker foam intermediate layers are of very low dielectric: constant material and are also designed to minimize the operative effect of the presence of these intermediate foam spacing layers. Thus, in the types of prior polarizers, as described, the arrays of polarizer elements (e.g., the meander-lines 14) are intended to produce the desired polarization change, and the support sheets and foam spacers are intended to have only minimal effects in the operation of the polarizer. As noted above, as the angle of incidence of an incident wave increases beyond a limited angular range, the performance of such prior polarizers rapidly degrades.
It is therefore an object of this invention to provide improved polarizers and, particularly, such polarizers usable with phased-array antennas to provide polarization conversion (e.g., linear to circular, vertical to horizontal, etc.) over a wide range of incidence angles.
Additional objects are to provide polarizers capable of performance over a wider range of incidence angles than prior devices, or capable of improved performance over a range of incidence angles within which prior devices are operable, or both.
Further objects are to provide antenna systems incorporating wide-angle polarizers, and new and improved polarizers which avoid disadvantages or limitations of prior devices.